A rheograph is an electronic instrument with an electrode sensor incorporated in it. It is used to measure the impedance of an area of biological tissue of the body and is a known device for studying physiological indexes of a human organism. In a general case, the sensor's electrodes may be electrically connected with each other and with outside instruments through various schemes of connections. An electrode connection through a conventional tetrapolar scheme is a preferred option. It is customary to use four metal electrodes two of which, named current-feeding electrodes or current-flow electrodes, serve to supply the measuring electric current to a monitored body area, while two other electrodes, named recording or measuring electrodes, serve to measure a voltage drop developing in the body area checked when the measuring current passes through it. The electrodes of the tetrapolar rheograph are usually made as metal rectangular plates applied onto the skin surface of the body area to be monitored, or as metal tapes attached on a plastic template, which is also applied onto the body area under study.
The disadvantage of the majority of the known tetrapolar rheographs consists in the presence of errors in the measurement of the impedance of biological tissue, caused by the fringe effects in the zone of measuring electrodes.
Another tetrapolar rheograph that is free of the above-described disadvantage employs annular current electrodes with the measuring electrodes placed in them. This design of a four-electrode sensor of the rheograph is the following.
The four-electrode sensor comprises two flat annular electrodes, placed on the skin of a human body area under study. A round measuring electrode is placed inside the ring of each current-feeding electrode. A pair of annular current-feeding electrodes is connected to a high-frequency generator via resistors, and a pair of round measuring electrodes is connected to the same generator via resistors and primary windings of the transformer. Because of such a connection, the round electrodes combine the functions of current-feeding and measuring electrodes, and annular electrodes serve as shielding electrodes. This provides the conditions for the formation of measuring current lines in the “volume” of biological tissue under study without dissipation thereof due to the fringe effects.
One of the limitations of the above-described sensor design is its consisting of two separate elements each of which contains a pair of electrodes arranged on a common base, namely, an annular electrode and a flat round electrode (inside of the former). When placed on a human body, each pair of electrodes is fastened independently, which fact cannot ensure an identity of the “electrode-skin” contacts for each pair that is required to obtain reliable measurement results. Additionally, the spaced-apart electrodes present no means of using them as a basis for making a small-sized and technological design. That is, those specific requirements that practice imposes upon the so-called “elements for biological information pickup” are not satisfied.
Other drawbacks of the above-described sensor are as follows. In some cases of making a decision concerning the functional state of human organism, it is necessary to use additional evidence of physiological indicators of other, non-rheographic modality, for example, temperature, pulse pressure in blood vessels, and so forth. These indicators are of particular interest when they are recorded from the same human body area in which the rheographic study is performed.